PBXs are increasingly being used in present day telephone systems. A PBX system ties together the telephones of an office, building or factory. Anyone within the PBX system can talk to someone else within the system without the cost and time of using outside lines and facilities.
Increasingly, PBX systems are becoming digital. The analog voice signals of a caller are converted into a digital representation. These digital signals are transmitted through the PBX system. Furthermore, PBX systems are increasingly used to transport computer data signals. This is due, in part, to the availability of personal computers in the home and office.
The heart of the system, the PBX switch as shown in FIG. 1, connects callers within the system, connects callers to outside lines if a call outside the PBX system is desired, and connects outside callers to lines within the system. A PBX switch generally has a number of modules or "line cards." Each line card is connected to a number of telephones or "terminals" and the line cards are connected to each other by a set of lines called a "bus", or sometimes, the "backplane bus." The bus, such as bus 40 in FIG. 1, has a timeslot bus. The timeslot bus carries the digital signals of a voice or the data of a computer, for example.
In a digital PBX, the voice signals are sampled at some rate, typically 8000 times per second (8 KHz), and the resulting voltage samples are converted into a digital representation, typically 8-bit ".mu.-law" or "A-law" encoding. The resulting sequence of bits (8000 times 8, or 64K bits/sec) is called the Pulse Code Modulation (PCM) representation of the original voice signal. The digital PBX transports and switches the PCM signals from place to place within the PBX system. Eventually, the PCM signals are converted back into an analog voice signal for a person to hear.
The PCM signals are carried on the bus 40 during particular time intervals, or timeslots. Each timeslot can carry the PCM 64K bit/second stream of data so that typically one timeslot is required for each incoming or outgoing voice path. Of course, a timeslot can also be used to carry computer data at rates up to 64K bits per second.
Besides a timeslot bus, the bus 40 has a signaling bus. Besides PCM-encoded voice signals and data signals, a digital PBX switch must also transport and switch signaling or control information associated with individual voice or data ports. For example, for a rotary-dial telephone it is important to know that the handset has been taken "off-hook," that a digit has been dialed, and so on. Thus, the PBX switch must have a way of gathering signaling information from individual voice ports, and transporting it to a control unit which acts upon this information by, for example, making voice connections.
Digital PBX switches are often connected in a network with other PBX switches and sources of PCM-encoded voice signals, for example, T1 digital trunk lines. Typically, each digital PBX switch or other PCM source has its own local oscillator which drives a local clock to provide an 8 KHz reference frequency for PCM sampling. However, the frequencies of the local oscillators may be different, albeit by amounts as small as 10 to 200 parts per million (ppm).
In a network environment, it is desirable to synchronize all PBX switches to a single 8 KHz reference clock, so that one PBX switch does not generate PCM signals faster than another switch. For example, if there were a 125 ppm difference in clock frequencies, one PBX switch would generate one PCM sample signal per second more than the other switch could absorb.
Thus, a PBX switch should provide a means of synchronizing its clock with some "master" clock source. In a typical PBX switch, this is done through costly, dedicated packaging and cabling arrangements that simply replace the local clock with a clock which is synchronized to a master clock source.
Another possible solution is a digital phase-locked loop (DPLL) circuit to synchronize the local clock with a second clock. However, a problem with DPLLs is that the local clock in following the second clock may require the circuits of the PBX to quicken their response time beyond their limits.
Furthermore, in prior art solutions of synchronizing the local clock, timing corrections can occur at any time. If these corrections are made while the PBX switch is performing some function, the chances of miscommunication are increased.
All these prior art clock synchronization solutions are localized. The signals of the second clock must be brought from the line card coupled to the second clock source to the central control module of the PBX, which bears the local clock and performs the synchronization. This requires the increased expense of separate and dedicated cabling between the control module and each of the line cards which may require synchronization.
The present invention solves or substantially mitigates these problems. The costly solution of dedicated packaging and cabling arrangements is avoided. Rather, the present invention provides for the synchronization of the local clock with a second clock without the problem of prior art DPLLs of requiring circuits to respond in times beyond their limits. Synchronization occurs at specific times to lower the chances of miscommunication. Clock synchronization in accordance with the present invention is distributed to allow any module in a PBX switch to force the local clock of the switch to adjust its frequency. This is done without the separate, dedicated modules and cabling for localized clock synchronization.